Knowledge about cannabis and its impacts is constantly evolving. The majority of people who use cannabis do not suffer any negative consequences. However, various factors seem to contribute to negative effects in some people.
Although problems may occur for first-time cannabis users, issues related to physical and mental health mainly arise as a result of repeated use over several months or years. Accidents and injuries, on the other hand, may occur as a result of one-time or occasional use. Certain factors can increase or reduce a cannabis user’s risk of being negatively affected:
- A personal or family history of mental health issues such as psychosis and bipolar disorder
- Frequency of use: Regular use (once a week or more) is often linked to increased risk of health problems
- Type of product used: Products with higher THC content may be more harmful
- The age at which a person starts to use cannabis: use during adolescence is generally associated with an increased risk of negative effects
- The circumstances of use, for example:
- When cannabis is combined with other substances such as alcohol or medications
- When an individual has personal responsibilities such as work or the supervision of children
- The method of use: Smoking cannabis appears to be more risky than other methods of use, although poisoning from foods containing cannabis is common among inexperienced users.
However, caution should be exercised before attributing the cause of a health problem to cannabis use. It is possible that the problem:
- Was already present prior to cannabis use
- Is a result of cannabis use
- Is influenced by cannabis use or, conversely, influences cannabis use
Based on these factors, it is difficult to predict whether or not an individual will experience significant problems after using cannabis. Most experts agree that cannabis use is never 100% safe.
Risks to cognitive abilities
The health risks and negative health impacts of cannabis use stem from its effects on cognitive abilities, such as:
- Attention span
- The ability to make decisions
These effects can impact daily activities such as:
- Driving a car
- Learning activities
- Other situations that require coordination and speed
Cannabis begins to take effect within minutes of being inhaled, and a little later if ingested. The effects often last for several hours and are usually reversible. Some studies suggest that reduced cognitive function may persist longer in the event of sustained and repeated use, especially if it begins in adolescence.
Physical health risks
Cannabis use can exacerbate some existing health problems, such as chronic diseases:
- Cannabis use increases the heart rate and can alter its rhythm. It also increases blood pressure.
- Inhaling cannabis smoke can aggravate existing respiratory diseases and even promote disease onset.
- Like tobacco smoke, cannabis smoke contains a number of substances that are harmful to your health, including some carcinogens.
- However, the most recent scientific studies do not prove that the risk of developing lung, throat, or neck cancer is higher among cannabis users.
Mental health risks
Individuals who are under the influence of cannabis may experience psychotic symptoms such as:
- Hallucinations with false visual, auditory, and/or tactile perceptions
- Paranoid ideas that seem detached from reality
In most cases, these psychotic experiences are limited to when the person is intoxicated and disappear on their own. Other individuals may experience persistent, long-term, and much more serious symptoms. According to experts, cannabis use does not cause psychotic disorders, but it can be a contributing factor for some people.
Individuals who regularly use cannabis may show a lack of interest in activities other than using cannabis (studies, work, leisure, etc.). They may also experience symptoms of depression, for example:
- Deep sadness
- Sense of worthlessness
Some individuals with depression may be tempted to use cannabis to relieve their symptoms. To date, scientific evidence has not shown that cannabis is effective in treating depression. Using it for this purpose is not recommended.
Some studies suggest that cannabis users experience symptoms of anxiety more frequently than non-users However, it is difficult to predict whether or not cannabis will influence their anxiety levels. Some people may experience panic attacks when they use cannabis, while others find it relaxing.
Problematic cannabis use
Cannabis use can be problematic if people lose control of their use and suffer negative consequences in various spheres of their life as a result. Some people may become addicted to cannabis, as it is the case with other substances. Individuals may:
- Develop a tolerance to the substance (i.e., need to use more to feel the same effect)
- Experience withdrawal symptoms when they reduce or stop use
- Have a strong desire to use
- Be unable to stop using
- Spend a significant portion of their time buying or using cannabis or recovering from cannabis use
- Use repeatedly in such a way that it prevents them from fulfilling important obligations at work, school, or home
- Use despite personal and/or social problems related to use
- Reduce or give up social, professional, or leisure activities as a result of use
About 1 in 11 people who use cannabis will develop a problematic use of cannabis in their lifetime. Among adolescents, 1 in 6 users will develop addiction problematic use. Use of and addiction to other products including alcohol and tobacco are more common among cannabis users.
Research has also established the existence of a cannabis withdrawal syndrome. It occurs when regular cannabis users significantly reduce or stop cannabis use. Symptoms include:
- Sleep problems, which may last a number of weeks after stopping use
Risks of Regular Use
The health risks of cannabis use increase with both the frequency (e.g., number of times used in a week) and duration of use (e.g., number of years used).
Risks of mixing cannabis with other substances
Cannabis and Alcohol
Whenever you drink alcohol or take a drug, the effect the substance will have on you is always a bit unknown. Mixing alcohol with recreational or prescription drugs increases the likelihood of harmful effects. That’s why a lot of cannabis users drink little or no alcohol when they consume cannabis. They know the two don’t mix.
Mixing cannabis with alcohol further reduces your ability to drive a vehicle.
Cannabis and Tobacco
Mixing cannabis and tobacco isn’t a good idea either. Their combined use presents a greater health risk—not to mention that tobacco is highly addictive.
Cannabis can have an impact on the effect of the medication you’re taking. The inverse is also true: some medications can alter the effects of cannabis. Before consuming cannabis, ask a health professional if there are any known interactions with your medications.
When a mother consumes cannabis, the THC winds up in the placenta she’s carrying and in her breast milk. Since the short- and long-term effects of THC exposure on fetuses and babies are currently unknown, it’s best to avoid cannabis and cannabis-derived products and exposure to secondhand smoke if you’re pregnant or breastfeeding. If you’re having a hard time giving up cannabis, talk to a trusted health professional.
For more information, visit the Alcohol or Other Drug Use During Pregnancy and Breastfeeding page page on the Gouvernement du Québec website.
AGRAWAL, Arpana et al. Cannabis involvement in individuals with Bipolar Disorder. Psychiatry Research, vol. 185, no 3, 2011, p. 459 - 461.
AMERICAN PSYCHIATRIC ASSOCIATION. Diagnostic and statistical manual of mental disorders (5th ed), 2013.
BÉLANGER, Richard E et al. Adding Tobacco to Cannabis—Its Frequency and Likely Implications. Nicotine & Tobacco Research, vol. 13, no 8, 2011, p. 746 - 750.
BOSSONG, Matthijs G. et Raymond NIESINK. Adolescent brain maturation, the endogenous cannabinoid system and the neurobiology of cannabis-induced schizophrenia. Progress in neurobiology, vol. 92, no 3, 2010, p. 370 - 385.
BUDNEY, Alan J et John R HUGHES. The cannabis withdrawal syndrome. Curr Opin Psychiatry, vol. 19, no 3, 2006, p. 233 - 238.
BUDNEY, Alan J et al. Marijuana Dependence and Its Treatment. Addiction Science & Clinical Practice, vol. 4, no 1, 2007, p. 4 -16.
CALABRIA, Bianca et al. Does cannabis use increase the risk of death? Systematic review of epidemiological evidence on adverse effects of cannabis use. Drug and Alcohol Review, vol. 29, no 3, 2010, p. 318 - 330.
CASCINI, Fidelia, Carola AIELLO et GianLuca DI TANNA. Increasing delta-9-tetrahydrocannabinol (Δ-9-THC) content in herbal cannabis over time: Systematic review and meta-analysis. Current Drug Abuse Reviews, vol. 5, no 1, 2012, p. 32 - 40.
CREAN, Rebecca D, Natania A CRANE et Barbara J MASON. An Evidence Based Review of Acute and Long-Term Effects of Cannabis Use on Executive Cognitive Functions. Journal of Addiction Medicine, vol. 5, no 1, 2011, p. 1 - 8.
CURRIE, Candace et al. (eds) Social determinants of health and well-being among young people. Health Behaviour in School-aged Children (HBSC) study: international report from the 2009/2010 survey. Health Policy for Children and Adolescents, No. 6. Copenhagen, Denmark: WHO Regional Office for Europe, 2012.
DEGENHARDT, Louisa et al. The persistence of the association between adolescent cannabis use and common mental disorders into young adulthood. Addiction, vol. 108, no 1, 2013, p. 124 - 133.
FISCHER, Benedik et al. Lower-Risk Cannabis Use Guidelines: A Comprehensive Update of Evidence and Recommendations. American Journal of Public Health, vol. 107, no 8, 2017, p. 1-12.
FREEMAN, Daniel et al. How Cannabis Causes Paranoia: Using the Intravenous Administration of ∆ 9 -Tetrahydrocannabinol (THC) to Identify Key Cognitive Mechanisms Leading to Paranoia. Schizophrenia Bulletin, vol. 41, no 2, 2015, p. 391-399.
GABLE, Robert S. Comparison of acute lethal toxicity of commonly abused psychoactive substances. Addiction, vol. 99, no 6, 2004, p. 686-696.
GEORGE, Tony et Franco VACCARINO (éd.). Toxicomanie au Canada : Les effets de la consommation de cannabis pendant l’adolescence, Ottawa (Ontario). Centre canadien de lutte contre les toxicomanies, 2015. (in french only)
GIBBS, Melanie et al. Cannabis use and mania symptoms: A systematic review and meta-analysis. Journal of Affective Disorders, vol. 171, 2015, p. 39-47.
GIEDD, Jay N et al. Brain development during childhood and adolescence: a longitudinal MRI study. Nature Neuroscience, vol. 2, no 10, 1999, p. 861- 863.
GRANT, N Christina et Richard E BÉLANGER. Canadian Paediatric Society, Adolescent Health Committee. Cannabis and Canada’s children and youth. Paediatr Child Health, vol. 22, no 2, 2017, p. 98 -102.
GOYAL, Hemant, Hamza H AWAD et Jalal K GHALI. Role of cannabis in cardiovascular disorders. Journal of Thoracic Disease, vol. 9, no 7, 2017, p. 2079 -2092.
HASIN, Deborah S et al. Prevalence and Correlates of DSM-5 Cannabis Use Disorder, 2012 2013: Findings from the National Epidemiologic Survey on Alcohol and Related Conditions-III. The American Journal of Psychiatry, vol. 173, no 6, 2016, p. 588 -599.
HURD, Yasmin L et al. Trajectory of Adolescent Cannabis Use on Addiction Vulnerability. Neuropharmacology, vol. 76, Part B, 2014, p. 416 - 424.
INCHLEY, Joanna Catherine et al. (eds) Growing up unequal: gender and socioeconomic differences in young people’s health and well-being. Health Behaviour in School-aged Children (HBSC) study: international report from the 2013/2014 survey. Health Policy for Children and Adolescents, no. 7. Denmark, WHO Regional Office for Europe, 2016.
JACKSON, Nicholas J et al. Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies. Proceedings of the National Academy of Sciences of the United States of America, vol. 113, no 5, 2016, p. 500 -508.
JOBIDON, Laurence et Didier JUTRAS-ASWAD. Le cannabis et ses effets délétères: pour un débat plus nuancé. Drogues, Santé et Société, vol. 16, no 1, 2018, p. 1-26. (in french only)
JEUNESSE SANS DROGUE CANADA. Parler cannabis : Savoir discuter avec son ado. (Sans date), 23 p. (in french only)
KARILA, Laurent et al. Acute and long-term effects of cannabis use: a review. Current Pharmaceutical Design, vol. 20, no 25, 2014, p. 4112 - 4118.
KEDZIOR, Karina Karolina et Lisa Tabata LAEBER. A positive association between anxiety disorders and cannabis use or cannabis use disorders in the general population - a meta-analysis of 31 studies. BMC Psychiatry, vol. 14, no 136, 2014.
LINDSEY, Wesley T, David STEWART et Darrell CHILDRESS. Drug interactions between common illicit drugs and prescription therapies. The American Journal of Drug and Alcohol Abuse, vol. 38, no 4, 2012, p. 334 - 343.
MARCONI, Arianna et al. Meta-analysis of the association between the level of cannabis use and risk of psychosis. Schizophrenia Bulletin, vol. 42, no 5, 2016, p. 1262 - 1269.
MARTINASEK, Mary P, Jamie B MCGROGAN et Alisha MAYSONET. A Systematic Review of the Respiratory Effects of Inhalational Marijuana. Respiratory Care, vol. 61, no 11, 2016, p. 1543 - 1551.
MAYET, Aurélie et al. Cannabis use stages as predictors of subsequent initiation with other illicit drugs among French adolescents: use of a multi-state model. Addictive Behaviors, vol. 37, no 2, 2012, p. 160 - 166.
MCKIERNAN, Anna et Katie FLEMING. Canadian Youth Perceptions on Cannabis, Ottawa, Ont.: Canadian Centre on Substance Abuse, 2017.
MEIER, Madeline H et al. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Sciences of the United States of America, vol. 109, no 40, 2012, p. 2657 - 2664.
MEIER, Madeline H et al. Which adolescents develop persistent substance dependence in adulthood? Using population-representative longitudinal data to inform universal risk assessment. Psychological Medicine, vol. 46, no 4, 2016, p. 877 - 889.
MERGLEN, Arnaud et al. Weekly sport practice and adolescent well-being. Archives of Disease of Childhood, vol. 99, no 3, 2014, p. 208 - 210.
MERLINE, Alicia, Justin JAGER et John E SCHULENBERG. Adolescent risk factors for adult alcohol use and abuse: Stability and change of predictive value across early and middle adulthood. Addiction, vol. 103(Suppl 1), 2008, p. 84 - 99.
MOORE, Theresa HM et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. The Lancet, vol. 370, no 9584, 2007, p. 319 - 328.
MORGAN, Celia JA et al. Harms and benefits associated with psychoactive drugs: findings of an international survey of active drug users. Journal of Psychopharmacology (Oxford, England), vol. 27, no 6, 2013, p. 497 - 506.
PAREKH, Jai D et al. Cannabinoid hyperemesis syndrome. BMJ Case Reports, 2016; 20 Jan 2016, bcr2015213620.
PATTON, George C et al. Cannabis use and mental health in young people: Cohort study. British Medical Journal, vol. 325, no 7374, 2002, p. 1195 - 1198.
PATTON, George C et al. Reverse gateways? Frequent cannabis use as a predictor of tobacco initiation and nicotine dependence. Addiction, vol. 100, no 10, 2005, p. 1518 - 1525.
RESNICK, Michael D et al. Protecting Adolescents From Harm. Findings From the National Longitudinal Study on Adolescent Health. JAMA, vol. 278, no 10, 1997, p. 823 - 832.
RUBINO, Tiziana, Erica ZAMBERLETTI et Daniela PAROLARO. Adolescent exposure to cannabis as a risk factor for psychiatric disorders. Journal of Psychopharmacology, vol. 26, no 1, 2012, p. 177 - 188.
SEDDON, Jennifer L. et al. Cannabis use is associated with increased psychotic symptoms and poorer psychosocial functioning in first-episode psychosis: A report from the UK National EDEN Study. Schizophrenia Bulletin, vol. 42, no 3, 2016, p. 619 - 625.
STATISTIQUE CANADA. Enquête canadienne sur le tabac, l'alcool et les drogues (ECTAD) : sommaire de 2015. (in french only)
TESSIER, Sébastien. L’usage de cannabis au Québec et au Canada : portrait et évolution. Institut national de santé publique du Québec, 2017. (in french only)
THOMAS, Gerald, Jillian FLIGHT, Krista RICHARD et Stéphane RACINE. Pour une typologie de la consommation de cannabis adaptée aux politiques canadiennes : analyse tirée de l’Enquête sur les toxicomanies au Canada de 2004. Ottawa (Ontario) : Centre canadien de lutte contre l’alcoolisme et les toxicomanies, 2006, p. 1-8. (in french only)
TRAORÉ, Issouf et al. Enquête québécoise sur le tabac, l’alcool, la drogue et le jeu chez les élèves du secondaire, 2013. Évolution des comportements au cours des 15 dernières années, Québec, Institut de la statistique du Québec, 2014, 208 p. (in french only)
WANG, George Sam et al. Unintentional Pediatric Exposures to Marijuana in Colorado, 2009-2015. JAMA Pediatrics, vol. 170, no 9, 2016, e160971.
WILSON, Celeste R et al. Are clinical impressions of adolescent substance use accurate? Pediatrics, vol. 114, no 5, 2004, p. 536 - 540. (en anglais seulement)
WORD HEALTH ORGANIZATION. The health and social effects of nonmedical cannabis use, 2016, p. 1-72.
YAMREUDEEWONG, Weeranuj et al. Probable interaction between warfarin and marijuana smoking. Annals of Pharmacotherapy, vol. 43, no 7-8, 2009, p. 1347 - 1353.
YOUNG, Matthew M et Rebecca A JESSEMAN. The impact of substance use disorders on hospital use. Ottawa, ON: Canadian Centre on Substance Abuse, 2014, p. 1-31.
Last update: February 6, 2019 10:12 AM